Guiding camera performance evaluation method for asteroid defense

Abstract

The invention discloses a guide camera performance evaluation method for asteroid defense, and belongs to the technical field of deep space exploration. The method comprises the following steps: calculating an apparent magnitude, and calculating to obtain the irradiance of a target asteroid based on the apparent magnitude in combination with a Peson formula; based on the stability of the satellite platform, the focal length of the guide camera, the pixel size of the detector and the spot size of point target detection, calculating the upper limit of point target detection identification integral time of the guide camera; with the point target detection pixel area array as a calculation unit, calculating the number of effective electrons in the area array under specified integration time; calculating a noise equivalent electron number by taking the point target detection pixel area array as a calculation unit; according to the target detection signal-to-noise ratio threshold value, the detectable limit star magnitude of the guide camera and the detectable distance of the target asteroid are obtained through calculation, or according to the surface image signal-to-noise ratio threshold value, the surface image integration time is obtained through calculation. The method can provide a certain reference for target asteroid selection, guidance camera overall index demonstration and the like of an asteroid defense task.

Description

Technical Field

[0001] The present invention belongs to the technical field of deep space exploration, and in particular relates to a method for evaluating the performance of a guidance camera used for asteroid defense. Background Art

[0002] Near-Earth asteroids refer to asteroids whose perihelion distance is less than 1.3AU. Among them, near-Earth asteroids with a closest distance to the Earth of less than 0.05AU and a diameter greater than 140 meters are called Potentially Hazardous Asteroids (PHA). There are a large number of near-Earth asteroids. Due to their small size, dim targets and difficulty in observation, although about 30,000 near-Earth asteroids have been catalogued in orbit, it is estimated that they only account for 1% of the actual number. Near-Earth asteroid impacts are extremely harmful. The scientific community generally believes that 65 million years ago, the impact of a near-Earth asteroid on the Earth led to the extinction of 75% of the world's species, including dinosaurs. Since records began, many regional asteroid impacts have caused great harm to human life.

[0003] In the asteroid defense mission, due to the limited accuracy of ground-based telescopes in measuring the orbit of target asteroids, the impactor needs to have autonomous target detection and identification capabilities. In the long-distance detection phase of the target asteroid, after the impactor detects and identifies the target asteroid based on the onboard guidance camera, it implements multiple maneuvers to ensure the impact effect. In order to ensure the maneuver time margin of the impactor, the guidance camera is required to have an excellent signal-to-noise ratio, improve the target detection limit magnitude, and advance the target detectable time and distance as much as possible. In the target asteroid flyby phase, the target observation presents a surface image, which is used for the impactor to select the impact point. The guidance camera is required to have a suitable exposure time and a large dynamic range to retain the details of the target image.

[0004] Compared with the star sensors of traditional spacecraft based on stellar target observation, the design and performance evaluation of the guidance camera used for asteroid defense missions face the following difficulties: First, the target asteroid in the asteroid defense mission is tens of meters in diameter and the target is very dim, while the satellite platform attitude stability limits the integration time. The guidance camera design focuses more on detecting limit stars and the target detectable time and distance evaluation. Secondly, in the asteroid defense mission, the target apparent magnitude changes with the distance between the impactor and the target asteroid, and the guidance camera also needs to evaluate the dynamic target observation effect. Also, when the impactor approaches the target, it is necessary to evaluate the integration time under a specified signal-to-noise ratio. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a performance evaluation method for asteroid defense guidance camera. When the impactor is far away from the asteroid, the guidance camera works in the point target detection and recognition working mode. On the basis of the existing general star camera signal-to-noise ratio evaluation method, by introducing physical quantities such as the absolute magnitude of the asteroid, the asteroid distance, the stability of the satellite platform, and the detector spot size, it is used to evaluate the long-distance asteroid target detection and recognition capability of the guidance camera; when the impactor is close to the asteroid, the close-range surface target observation performance of the guidance camera is evaluated through formula transformation.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for evaluating the performance of a guidance camera for asteroid defense, the method comprising:

[0008] Step 1: Calculate the apparent magnitude of the target asteroid based on its absolute magnitude, albedo, solar phase angle, distance between the impactor and the target asteroid, and distance between the sun and the target asteroid. Calculate the irradiance of the target asteroid based on the apparent magnitude and the Posen formula.

[0009] Step 2: When detecting and identifying long-distance point targets, the guidance camera uses n n detector pixel arrays are used for point target detection, n≥2; when observing close-range surface targets, n=1. The upper limit of the integral time for point target detection and recognition by the guidance camera is calculated based on the stability of the satellite platform, the focal length of the guidance camera, the size of the detector, and the spot size of the point target.

[0010] Step 3: Taking the point target detection pixel array as the calculation unit, introducing the target irradiance, the guidance camera aperture, the system transmittance, and the detector quantum efficiency, calculate the number of effective electrons in the array under the specified integration time upper limit condition;

[0011] Step 4, taking the point target detection pixel array as the calculation unit, respectively calculate the noise equivalent electron number under the comprehensive effect of quantization noise, readout noise, dark noise, and shot noise;

[0012] Step 5: According to the target detection signal-to-noise ratio threshold, the detection limit visual magnitude of the guidance camera and the detectable distance of the target asteroid are calculated respectively; or the surface image integration time is calculated according to the surface image signal-to-noise ratio threshold.

[0013] The beneficial effects of the present invention are:

[0014] The present invention has established a full-chain target observation performance evaluation method that includes many characteristics such as the target, the impactor platform, and the guidance camera payload. The mission status covers long-distance point target detection and identification and close-range surface target observation. Its good versatility ensures the applicability of asteroid defense missions with uncertain impact target characteristics. The present invention can also provide a certain reference for the selection of target asteroids for asteroid defense missions, the overall design of satellites, and the demonstration of the overall indicators of guidance cameras. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a flow chart of a method for evaluating the performance of a guidance camera for asteroid defense according to the present invention;

[0016] Figure 2 This is a schematic diagram of the relative positions of the asteroid, the sun, and the impactor;

[0017] Figure 3 Schematic diagram of the relative positions of the point target detection array and the diffuse light spot. DETAILED DESCRIPTION

[0018] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0019] like Figure 1 FIG. 1 is a flow chart of a method for evaluating the performance of a guidance camera for asteroid defense according to the present invention, which specifically includes the following steps:

[0020] Step 1: Calculate the characteristics of the asteroid. First, calculate the apparent magnitude based on the absolute magnitude, albedo, solar phase angle, distance between the impactor and the target asteroid, and distance between the sun and the target asteroid. Then, calculate the irradiance of the target asteroid based on the apparent magnitude and the Posen formula.

[0021] Step 2: Calculate the integration time of the guidance camera. When detecting and identifying long-distance point targets, the guidance camera uses n n detector pixel arrays are used for point target detection to increase detection energy acquisition, thereby improving the signal-to-noise ratio of target detection and identification. n is the number of rows and columns of the pixel array used for detection and identification. When observing and identifying long-distance point targets, n ≥ 2, and when observing close-range surface targets, n = 1. By introducing the stability of the satellite platform, the focal length of the guidance camera, the size of the detector, and the spot size of the point target, the upper limit of the integration time for the guidance camera point target detection and identification is calculated;

[0022] Step 3: Calculate the effective signal of the guidance camera, taking the point target detection pixel array as the calculation unit, and calculate the effective number of electrons in the array under the specified integration time upper limit condition by introducing physical parameters such as the guidance camera aperture, system transmittance, and detector quantum efficiency.

[0023] Step 4: Calculate the guidance camera noise, taking the point target detection pixel array as the calculation unit, and calculate the noise equivalent electron number under the combined effect of shot noise, readout noise, dark noise, quantization noise and other noise items.

[0024] Step 5: Evaluate the performance of the guidance camera. According to the target detection signal-to-noise ratio threshold, important information such as the detection limit visual magnitude of the guidance camera and the detectable distance of the target asteroid can be calculated; or according to the surface image signal-to-noise ratio threshold, the surface image integration time can be calculated.

[0025] Specifically, in step 1, the relative position relationship between the asteroid, the sun and the impactor is as follows: Figure 2 shown.

[0026] Apparent magnitude of asteroids The calculation is about the absolute magnitude of the asteroid , sun-star distance r, instrument-star distance l and solar phase angle This method uses the widely used two-parameter Magnitude system as a model for the magnitude of asteroids.

[0027] (1)

[0028] in, To describe the slope parameter of the asteroid surface material's scattering characteristics of sunlight, it is usually assumed that , and is the phase function;

[0029] (2)

[0030] Among them, A1=3.332, A2=1.862, B1=0.631, B2=1.218.

[0031] In the absence of absolute magnitude of asteroids When , it can be calculated by the diameter and albedo of the asteroid, and then the apparent magnitude of the asteroid can be obtained according to formula (1);

[0032] (3)

[0033] Among them, D is the diameter of the asteroid; P is the surface albedo of the asteroid, which can generally be taken as 0.14.

[0034] Corresponding spectral irradiance of asteroids According to the apparent magnitude of the asteroid , solar magnitude Corresponding spectral irradiance to the sun , calculated by the Posen formula:

[0035] (4)

[0036] In step 2, the relative position relationship between the detector pixel recognition array and the point target observation diffuse spot is as follows: Figure 3 shown.

[0037] In the long-distance point target detection working mode, calculate the upper limit of the navigation sensor integration time. At this time, the guidance camera's detector is in n In the n-pixel array recognition working mode, due to the instability of the satellite platform attitude, in order to ensure point target detection and recognition, the displacement range of the imaging spot is controlled within the integration time to be within n n pixel array.

[0038] At this time, the impactor platform attitude changes angle , the field of view angle corresponding to the diffuse spot observed by the asteroid point target camera The field of view angle corresponding to the target recognition array The following relationship should be satisfied:

[0039] (5)

[0040] The field of view angle corresponding to the diffuse spot observed by the asteroid point target camera The angular resolution of the guidance camera is the result of the optical design of the guidance camera, which can be obtained by measuring the diffuse spot diameter. and the focal length of the camera The calculation results are:

[0041] (6)

[0042] Target recognition array corresponding to the field of view angle , can be obtained by using the single pixel size and camera focal length The calculation results are:

[0043] (7)

[0044] Points time limit The angle of impactor platform attitude change and platform stability The calculation results are:

[0045] (8)

[0046] In step 3, the spectral radiation flux received by the guide camera detector under a certain aperture is first calculated according to the illumination corresponding to the apparent magnitude, so as to calculate the number of effective signal equivalent electrons received within the upper limit of the integration time.

[0047] The spectral flux reaching the guidance camera detector is calculated as follows:

[0048] (9)

[0049] in, To guide the camera's system transmittance, To guide the camera system occlusion rate, The effective aperture of the guidance camera.

[0050] The number of electrons generated by the target asteroid radiation received by the guidance camera within the upper limit of the integration time is:

[0051] (10)

[0052] in, The number of electrons generated by the target asteroid radiation received by the guidance camera is the number of electrons equivalent to the effective signal. is the single photon energy, is the detector quantum efficiency, is the wavelength, is Planck's constant, The speed of light.

[0053] In step 4, the guidance camera quantizes the electronic analog signal generated by the observed target asteroid into a digital signal. The noise items mainly include shot noise, readout noise, dark current noise and quantization noise. Shot noise is related to the Poisson distribution of photons, and its magnitude is equal to the standard deviation of the number of signal electrons; dark noise is the noise generated by the thermal motion of the detector carriers, which also presents a Poisson distribution and is positively correlated with the integration time; readout noise is noise related to the detector design and is superimposed on the effective signal; quantization noise is the noise factor caused by the photon quantization error when the detector completes the analog-to-digital conversion, which is related to the electronic gain and the number of quantization bits. n for point target recognition The number of noise electrons in an n-pixel array is:

[0054] (11)

[0055] (12)

[0056] in, is the noise equivalent electron number; It is single pixel dark noise. Due to the use of area array recognition mode, its total dark noise increases accordingly. It is single pixel readout noise and has nothing to do with the working mode; is the quantization noise; is the saturated electron number; For electronics gain; The number of bits for detector quantization.

[0057] In step 5, based on the guidance camera target detection signal-to-noise ratio evaluation calculation, a guidance camera performance evaluation method that can be used for asteroid defense is established, which is related to the full chain of factors such as the magnitude of the target asteroid, the distance between the impactor and the asteroid, the stability of the satellite platform, the guidance camera aperture, the guidance camera focal length, the detector quantization efficiency, and the detector pixel size.

[0058] The detector image signal-to-noise ratio and the target recognition image signal-to-noise ratio threshold are used as input when detecting and identifying asteroid point targets at long distances (usually the threshold for ground processing of the original image is 3:1, and the threshold for autonomous recognition on the satellite is 5:1), and the detectable recognition distance for the specified asteroid is calculated. When observing asteroid surface targets at close range, the image signal-to-noise ratio threshold is used as input (it can be 100:1), and the integration time adjustment is calculated as the target approaches.

[0059] (13)

[0060] The present invention can be used to evaluate the performance of the guidance camera used for asteroid defense. Based on the background of kinetic impact of the impactor in the asteroid defense mission, a full-chain target observation performance evaluation method is established that includes many characteristics of the target asteroid, the impactor platform, and the guidance camera payload. In the implementation of the specific performance evaluation method, by giving the scattering characteristics of the target asteroid, the solar phase angle, the attitude stability of the satellite platform, the transmittance of the guidance camera system, the focal length, the detector pixel size, the spot size and other parameters, the signal-to-noise ratio of the guidance camera observing the target asteroid at a certain distance is calculated, and this is used as the criterion for the guidance camera performance evaluation. The target observable distance directly determines the impactor maneuvering time margin and maneuvering plan formulation in the asteroid defense mission, and plays a decisive role in the success or failure of the mission. In addition, this method can be further used for the calculation of other key parameters in the implementation plan of the asteroid defense mission through formula evolution, such as the calculation of the detectable limit apparent magnitude, the detectable distance of the target asteroid, the surface image integration time, etc. For example, for the assessment of the detectable limit visual magnitude, the observable distance of the guidance camera to the target asteroid can be calculated based on the scattering characteristics of the target asteroid, the solar phase angle, the satellite platform attitude stability, the guidance camera system transmittance, focal length, detector pixel size, spot size and other parameters as well as the target observation signal-to-noise ratio threshold, and then the target detection limit visual magnitude can be converted.

[0061] The present invention establishes a full-chain target observation performance evaluation method that includes many characteristics of the target, platform, and guidance camera payload. Its good versatility ensures the applicability of asteroid defense missions with uncertain impact target characteristics. The present invention can also provide a certain reference for the selection of target asteroids for asteroid defense missions, the overall design of satellites, and the demonstration of the overall indicators of guidance cameras.

[0062] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for evaluating the performance of a guidance camera for asteroid defense, characterized in that: The method comprises: Step 1: Calculate the apparent magnitude of the target asteroid based on its absolute magnitude, albedo, solar phase angle, distance between the impactor and the target asteroid, and distance between the sun and the target asteroid. Calculate the irradiance of the target asteroid based on the apparent magnitude and the Posen formula. Step 2: When detecting and identifying long-distance point targets, the guidance camera uses n n detector pixel arrays are used for point target detection, n≥2; when observing close-range surface targets, n=1. The upper limit of the integral time for point target detection and recognition by the guidance camera is calculated based on the stability of the satellite platform, the focal length of the guidance camera, the size of the detector, and the spot size of the point target. Step 3: Taking the point target detection pixel array as the calculation unit, introducing the target irradiance, the guidance camera aperture, the system transmittance, and the detector quantum efficiency, calculate the number of effective electrons in the array under the specified integration time upper limit condition; Step 4, taking the point target detection pixel array as the calculation unit, respectively calculate the noise equivalent electron number under the comprehensive effect of quantization noise, readout noise, dark noise, and shot noise; Step 5: According to the target detection signal-to-noise ratio threshold, the detection limit visual magnitude of the guidance camera and the detectable distance of the target asteroid are calculated respectively; or the surface image integration time is calculated according to the surface image signal-to-noise ratio threshold.

2. A method for evaluating the performance of a guidance camera for asteroid defense according to claim 1, characterized in that: The step 1 comprises calculating the apparent magnitude of the asteroid according to the following formula (1): : （1） in, represents the absolute magnitude of the asteroid, Indicates the distance between the sun and the star. Indicates the distance to the star. represents the solar phase angle, To describe the slope parameter of the asteroid surface material's scattering characteristics of sunlight, and is the phase function; （2） in, =3.332, =1.862, =0.631, =1.218; Absolute magnitude of asteroid Calculated as follows: （3） Where D is the diameter of the asteroid; P is the surface albedo of the asteroid; Calculate the spectral irradiance of asteroids : （4） In the formula, represents the apparent magnitude of the Sun, Represents the spectral irradiance of the sun.

3. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 1, characterized in that: In step 2, in order to ensure the detection and recognition of long-distance point targets, under the condition of preset attitude stability of the satellite platform, the displacement range of the imaging spot is controlled to be within n within the integration time. n-pixel array, impactor platform attitude change angle , the field of view angle corresponding to the imaging spot of the asteroid point target The field of view angle corresponding to the target recognition array The following relations are satisfied: （5） Among them, the field angle corresponding to the imaging spot of the asteroid point target is , is the result of the optical design of the guiding camera, and the spot diameter is measured and the focal length of the camera The calculation results are: （6） Target recognition array corresponding to the field of view angle , through the single pixel size And the camera focal length f is calculated, and when n=1, it is the angular resolution of the guidance camera: （7） Point target detection and recognition integration time limit The angle of impactor platform attitude change and platform stability The calculation results are: （8）。 4. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 3, characterized in that: The step 3 comprises: Spectral flux reaching the target detection pixel array of the guidance camera detector The calculation is as follows: （9） in, To guide the camera's system transmittance, To guide the camera system occlusion rate, The effective aperture of the guidance camera; The number of electrons generated by the target asteroid radiation received by the guidance camera within the upper limit of the integration time is: （10） in, is the number of effective target asteroid photons received by the guidance camera, is the single photon energy, is the detector quantum efficiency, is the wavelength, is Planck's constant, The speed of light.

5. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 4, characterized in that: In step 4, n is used for point target detection. The noise equivalent electron number in the n-pixel array is: （11） （12） in, is the noise equivalent electron number; is single pixel dark noise; It is the single pixel readout noise; is the quantization noise; is the saturated electron number; For electronics gain; The number of bits for detector quantization.

6. The method for evaluating the performance of a guidance camera for asteroid defense according to claim 1, characterized in that: The target detection signal-to-noise ratio in step 5 is calculated according to the following formula: （13）。

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